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Nickel Arsenides

The freezing point curve of Ni-As alloys containing up to 57.4 per cent. As has been studied. The freezing point of nickel is lowered by addition of arsenic until with 27.8 per cent, of the latter a eutectic point is reached at 900° C. With 34.3 per cent, of arsenic there is a maximum at 998° C., corresponding with the arsenide Ni5As2. A second eutectic point is reached at 804° C. with 43.3 per cent, of arsenic, and the curve then rises to another maximum at 968° C. and 56.0 per cent, of arsenic, corresponding with the arsenide NiAs. Solid solutions are formed between the limits 0 to 5.5 per cent, and 33.5 to 35.7 per cent, of arsenic. Alloys containing 35.7 to 56 per cent, of arsenic undergo a transformation on cooling, with evolution of heat, the maximum development occurring at 45.5 per cent, of arsenic, thus indicating the formation of a third arsenide, Ni3As2. In addition, the diarsenide, NiAs2, is known to exist, and a product obtained by heating nickel in arsenic trichloride vapour approximated in composition to the hemi-arsenide, Ni2As, but its identity as a definite compound has not been confirmed.

Pentanickel Di-arsenide, Ni5As2

Pentanickel Di-arsenide, Ni5As2, prepared by fusing together the elements in the required proportions, is a hard, brittle compound. It is non-magnetic.

Triniekel Di-arsenide, Ni3As2

Triniekel Di-arsenide, Ni3As2, is formed by reducing nickel arsenate, or a mixture of nickel oxide and arsenious oxide, with fused potassium cyanide; when reduced nickel is heated at 800° to 1400° C. in vaporised arsenic chloride; or when an alcoholic solution of nickel ammonium sulphate is dropped into an atmosphere of arsine completely free from air, the reaction vessel being repeatedly shaken. The arsenide is a grey substance which melts at 1000° C. Its density is 7.86 at 0° C. The crystals are tetragonal with a = 3.45 A. and c = 21.7 A., and a superposed 2-dimensional overstructure having a = 6.9 A. The arsenide is very stable and is only slightly decomposed at 1400° C. It is attacked when heated in oxygen, chlorine or sulphur vapour. It readily dissolves in acids and is attacked more slowly by alkalis. According to Descamps, when heated under fused boric oxide it loses arsenic to yield an arsenide Ni3As, but the existence of this arsenide has not been confirmed.

Nickel Monarsenide, NiAs

Nickel Monarsenide, NiAs, is formed when reduced nickel is heated at 400° to 800° C. in arsenic chloride vapour. It is identical with the mineral niceolite or nickeline, and has, like it, a red colour. Its density is 7.57 at 0° C. When heated it commences to lose arsenic at 100° C. and is gradually converted into Ni3As2. The crystal structure of nickeline has been examined, but the results obtained are not concordant.

Nickel Di-arsenide, NiAs2

Nickel Di-arsenide, NiAs2, occurs in Nature as chloanthite and rammelsbergite. It may be prepared by heating either of the two previously described arsenides with arsenic in an atmosphere of hydrogen at decreasing temperatures; the arsenic is absorbed and the di-arsenide forms as a grey powder as the temperature falls below 400° C.

Rammelsbergite is readily oxidised by aerated waters, the oxidation proceeding much more rapidly than in the case of niccolite and other arsenide minerals; the oxidation is accelerated by the presence of iron pyrites, manganese dioxide or platinum black.

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